Feat run param sep

[jmccreight]

This PR

1. separates NHM/PRMS parameters using the Parameter class.
2. Introduces dis_hru and dis_seg for the nhm as special "discretization" subsets of the NHM/PRMS
3. Keeps Model backwards compatible with a single PrmsParameter object being passed
4. Updates Model to take a list of discretizations and processes along with their individual data.

• Closes Run NHM processes with separate parameters #189 and Introduce Discretizations to Model() #190
• Tests and/or performance benchmarks added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst or it's sub rsts?

[codecov-commenter]

Codecov Report

Merging #188 (32d905e) into develop (b86e4bb) will increase coverage by 2.17%.
The diff coverage is 94.62%.

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@@             Coverage Diff             @@
##           develop     #188      +/-   ##
===========================================
+ Coverage    77.34%   79.52%   +2.17%     
===========================================
  Files           50       51       +1     
  Lines         6364     6421      +57     
===========================================
+ Hits          4922     5106     +184     
+ Misses        1442     1315     -127     

Impacted Files	Coverage Δ
pywatershed/analysis/model_graph.py	11.01% <25.00%> (+0.58%)	⬆️
pywatershed/parameters/starfit_parameters.py	28.12% <40.00%> (+2.31%)	⬆️
pywatershed/hydrology/starfit.py	29.76% <80.00%> (ø)
pywatershed/utils/path.py	81.81% <81.81%> (ø)
pywatershed/base/parameters.py	90.00% <86.36%> (-3.11%)	⬇️
pywatershed/base/conservative_process.py	89.85% <89.85%> (ø)
pywatershed/base/control.py	92.80% <92.00%> (-0.54%)	⬇️
pywatershed/hydrology/prms_snow.py	90.45% <94.11%> (ø)
pywatershed/hydrology/prms_soilzone.py	88.19% <95.00%> (ø)
pywatershed/base/model.py	94.73% <95.12%> (-0.51%)	⬇️
... and 22 more

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[jmccreight]

Does this address #145?